The dynamic response range of a television camera tube is limited and above this range the camera tube overloads. This causes a problem when the tube is used to view a scene, the brightness of at least a part of which is above the maximum value of the dynamic range. This overloading can be compared to the response of the human eye. When a part of a scene which is being viewed is very bright, the iris contracts, thereby reducing the total amount of light reaching the retina. Unfortunately, this has the effect of darkening the image of the less bright parts of the scene, leading to a loss of contrast in these darker parts with a consequent loss of definition. This contraction of the iris may continue until only the highlights are visible and the definition in the darker scenes is lost; if the light from the highlight regions is still excessive, no detail is visible in the highlight regions either.
Television cameras may similarly be provided with an automatically operating iris. Typically, the light content of a scene is sampled from time to time and the average value is determined. This is compared with a stored average value and the camera output is attenuated to bring the actual average value within the pre-set limits of the stored average. The attenuation may be achieved electronically, by altering the target voltage, or mechanically by adjusting the iris setting. The automatic adjustment is, however, not as efficient as that of the human eye, which is in itself inefficient in certain conditions, as described above.